I've been a Firefox user since before it was called Firefox, and also before it was called Firebird. I started using Firefox with Mozilla Phoenix.
Before Phoenix, Mozilla had an entire suite of internet applications: a browser, a mail client, an email program, and an HTML Composer. The suite was large, annoying to have to install to use the web browser, and the web browser was also slow. The Phoenix project was an attempt to break just the web browser out of the suite, and to create a fast lightweight browser with modern standards support.
The experiment worked…for a while. I switched to Firefox when it was the fastest and best. Unfortunately over time Firefox has gotten more and more bloated with features, and I find it quite slow. My switch to Chrome was due to a marginally nicer UI, but most importantly because Firefox had become just as unwieldy as the old Mozilla browser.
Over time Chrome has become bloated too, so now I use a little bit of Chrome, a little bit of Firefox, and a little bit of Safari depending on what I'm doing.
Chrome feels like home, Firefox feels familiar but awkward right now, and Safari is fast and had (until recently) awful keyboard shortcuts. Now it's usable :)